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University at Albany chemists have created a new high-energy compound that could revolutionize rocket fuel and make space flights more efficient. Upon ignition, the compound releases more energy relative to its weight and volume compared to current fuels. In a rocket, this would mean less fuel required to power the same flight duration or payload and more room for mission-critical supplies. Their is published in the Journal of the American Chemical Society.

"In rocket ships, space is at a premium," said Assistant Professor of Chemistry Michael Yeung, whose lab led the work. "Every inch must be packed efficiently, and everything onboard needs to be as light as possible. Creating more efficient fuel using our new compound would mean less space is needed for fuel storage, freeing up room for equipment, including instruments used for research. On the return voyage, this could mean more space is available to bring samples home."

The newly synthesized compound, manganese diboride (MnB₂), is over 20% more energetic by weight and about 150% more energetic by volume compared to the aluminum currently used in solid rocket boosters. Despite being highly energetic, it is also very safe and will only combust when it meets an ignition agent like kerosene.

The underlying boron-based structure is also versatile; related research in the Yeung lab has demonstrated its potential to help build more durable for cars and serve as a catalyst for breaking down plastics.

It takes heat to make heat

Manganese diboride belongs to a class of chemical compounds thought to have unusual properties, yet exploring what exactly these properties entail has been limited by an inability to actually produce the compound.

"Diborides first started getting attention in the 1960s," said UAlbany Ph.D. student Joseph Doane, who works with Yeung. "Since these initial looks, new technologies are allowing us to actually synthesize chemical compounds that were once only hypothesized to exist.

"Knowing what we do about the elements on the periodic table, we suspected that manganese diboride would be structurally asymmetrical and unstable—factors which together would make it highly energetic—but until recently, we couldn't test it because it couldn't be made. Successfully synthesizing pure manganese diboride is an exciting achievement in and of itself. And now, we can test it experimentally and discover new ways to put it to use."

Synthesizing manganese diboride requires extreme heat generated using a tool called an "arc melter." The first step involves pressing manganese and boron powders together into a pellet, which is placed in a small, reinforced glass chamber. The arc melter trains a narrow electrical current on the pellet, heating it to a scorching 3,000°C (over 5,000°F). The molten material is then rapidly cooled to lock the structure in place.

At the atomic level, this process forces a central manganese atom to bond to too many other atoms, making for an overly crowded structure packed tight like a coiled spring.

3...2...1... Deformation

When exploring new chemical compounds, being able to physically produce the compound is critical. You also need to be able to define its molecular structure, in order to better understand why it behaves the way it does.

UAlbany Ph.D. student Gregory John, who works with computational chemist Alan Chen, built computer models to visualize manganese diboride's molecular structure. These models revealed something critical: a subtle skew, known as "deformation," which gives the compound its high potential energy.

"Our model of the manganese diboride compound looks like a cross section of an ice cream sandwich, where the outer cookies are made of a lattice structure comprised of interlocking hexagons," said John.

"When you look closely, you can see that the hexagons aren't perfectly symmetrical; they're all a little skewed. This is what we call 'deformation.' By measuring the degree of deformation, we can use that measure as a proxy to determine the amount of energy stored in the material. That skew is where the energy is stored."

Here's another way to picture it.

"Imagine a flat trampoline; there's no energy there when it's flat," said Yeung. "If I put a gigantic weight in the center of the trampoline, it will stretch. That stretch represents energy being stored by the trampoline, which it will release when the object is removed. When our compound ignites, it's like removing the weight from the trampoline and the energy is released."

New materials need new compounds

"There's this consensus among chemists that boron-based compounds should have unusual properties that make them behave unlike any other existing compounds," said Associate Professor of Chemistry Alan Chen.

"There's an ongoing quest to figure out what those properties and behaviors are. This sort of pursuit is at the heart of materials chemistry, where creating harder, stronger more extreme materials requires forging brand-new chemicals. This is what the Yeung lab is doing—with findings that could improve rocket fuel, catalytic converters and even processes for recycling plastics.

"This study is also a great example of the scientific process, where researchers pursue interesting chemical properties even when they're not certain what specific applications might emerge. Sometimes, present case included, the results are serendipitous."

Yeung's interest in boron compounds started when he was a grad student at the University of California, Los Angeles. His project was aiming to discover compounds harder than diamond.

"I distinctly remember the first time I made a compound related to manganese diboride," Yeung said. "There I was, holding this new material that was supposed to be super hard. Instead, it started to get hot and changed into a pretty orange color. I thought, 'Why is it orange? Why is it glowing? It shouldn't be glowing!' That's when I realized how energetic boron compounds can be. I put a pin in it to explore in the future, and that's exactly what we are working on now."